One of the goals for any cellular mobile communication system is to make use of radio resources as much as possible and to provide more and better services. Multiuser detection can obviously raise system performance and capacity, and comparing with the convention RAKE receiver, the spectrum efficiency is almost double.
Nevertheless, computation loads of the multiuser detection are huge and in direct proportion to square of the number of subscribers. It is impossible for the present microprocessor and Field Programmable Logic Array (FPLA) to perform such computation loads. Therefore, in 3GPP two systems: the wide-bandwidth time division duplex (WB-TDD) and narrow-bandwidth time division duplex (NB-TDD), where the multiuser detection is definitely used, the maximum length of spreading code (or spreading factor) in a time slot is 16, thereby the maximum number of subscribers is 16, and the length of scrambling code of local cell, i.e. cell-code for cell identification is 16 too.
The main purposes of the cell-code specified in the CDMA TDD standard are to counteract interference from adjacent cells and to whiten the signals from adjacent cells. This cell-code is different with the long code and short code used in other CDMA systems. In those CDMA systems, all subscribers share one long code and one short code, subscribers are differentiated by different phases of the long code and cells are differentiated by different phases of the short code. However, each cell has its own cell-code. For example, the NB-TDD system has 128 cell-codes.
Since the length of the spreading code and the cell-code of cells are all 16, the length of the spreading modulation code generated by them is 16 too. When there are 128 cell-codes in a system (the system can support 128 cells simultaneously), and each cell-code makes dot produce with 16 WALSH codes, then the system generates 16×128=2048 spreading modulation codes (a spreading modulation code is made by dot produce the WALSH code and the cell-code). In other words, there are 2048 spreading modulation codes with length 16. It is very difficult to guarantee that there are not only 128 groups of orthogonal codes in 2048 spreading modulation codes but also better cross-correlation of different code groups.
Selection of the cell-code length should be taken into account; if length of the cell-code is too short, it is a disadvantage to counteract interference of adjacent cells and is impossible to whiten signals. Taking the NB-TDD system as an example, there are 128 cell-codes, and it is very difficult to have good cross-correlation between these code groups. At present, although from the statistical point of view the cross-correlation properties between codes are better, but correlation between some code groups is very high or even completely correlative. For example, in NB-TDD, the first code and the 126th code are as follows:
Code 111111−11−11−1−1111−1−1Code 1261111−11−11−111−111−1−1
When Code 1 makes dot product with WALSH 12, the spreading modulation code is:                1 1 1 1 −1 1 −1 1 −1 1 1 −1 1 1 −1 −1.        
When Code 126 makes dot product with WALSH 0, the spreading modulation code is:                1 1 1 1 −1 1 −1 1 −1 1 1 −1 1 1 −1 −1.        
It is seen that these two spreading modulation codes are identical, and this is called repetition codes in the system.
As viewed from above instance, with correlation between some codes being very high or even completely correlative, there are some repetition codes in 2048 spreading modulation codes. The repetition code in a CDMA system is a disaster, especially when two subscribers at adjacent cells are allocated with the repetition code. In this case, these two different subscribers have the same spreading modulation code with length 16; only their midamble codes are different. Although these two subscribers can be differentiated by the midamble code, when the subscriber signals come from the same direction (signals from any direction in case of a receiver with omni-antenna), their strong paths are basically coincidence during demodulation, so it is impossible to differentiate the subscribers with the midamble code. With the smart antenna and the code allocation algorithm, the interference can be suppressed in certain degree, but it cannot be completely eliminated. Once different subscriber signals are spread and modulated with the same spreading modulation code, and they arrive the demodulation end at the same time, then strong interference will appear at the receiver. It is almost definitely to say that in this case the receiver cannot correctly demodulate the received signals so that the spreading gain disappears and the system cannot work normally at an identical frequency.
Furthermore, amplitude-frequency characteristic in the band of signals is worse, and cannot satisfy the whitening requirement, so the demodulation is difficult.
There are three solution for the above problem: first solution, to change the scrambling code i.e. cell-code to make that there is no repetition code in the 2048 spreading modulation codes; second solution, with the smart antenna and the code allocation algorithm, to guarantee that no repetition code is used simultaneously at adjacent cells; third solution, to keep longest length of the spreading code being 16 and the maximum number of subscribers being 16, but a long cell-code, such as length of 32, 64 or 128 (a multiple of 16) using for differentiating signals from different cells.
With the first solution, it is necessary to look for new cell-codes, but even though these scrambling codes can be found, it is difficult to change the connatural cross-correlation between codes (because some codes are not completely correlation, but they have high correlation).
With the second solution, the repetition code problem can be avoided in certain degree, but cannot be eliminated completely, and the impact of high correlation codes cannot be avoided.
With the third solution, the repetition code problem is mostly overcome, and the signals are whitened with the long cell-code; this makes that spectrum of the modulated spreading signals becomes more flat, and the peak-average power ratio is decreased, so that requirement to filter performance is reduced and radio frequency system performance is raised. Nevertheless, with this solution, signal-processing method needs to be changed and the computation loads are increased.
When the long cell-code is applied at the transmitter and joint detection is still made at the receiver, the computation loads will be much more increased. Up till now, there is no any disclosed method to implement this solution.